i think you guys have scared away any new fish keepers that read this forum lol
Anyways, don't keep the clam in there for too long cuz they will rot and with a fish as sensitive as the moorish idol, you really want to keep the water condition as good as possible.
And yes the moorish idol is quite difficult to keep but it can be done. They DO require large tanks, but again, it can be done in smaller tanks. I have a very experienced friend who has 3 in a ~220 gallon that he has had for 2 years. He started out with five though, but still not bad. Don't know what his secret is though Sorry!
This is how he got them to eat substitute foods:
If you are lucky enough to get an individual that eats clams which you are, try adding small bits of frozen foods squished onto the clam. Over time they will try it out and hopefully will like it. Sounds simple enough? don't know if it works but thats how he did it. He also said that they like huge flakes of spirulina. They are more willing to chew bits from a giant peice then to eat little bits of flakes floating around even if they are the exact same thing. I once considered stocking my whole 385 with only moorish idols but never took the chance since i have to save a LOT of lunch money to buy something that might not survive
If your tank is big enough they are supposed to do much better in groups of 3-4. Good luck!!!